Chemical Resale of Santa Barbara (CRSB) offers a variety of chemicals to
independent researchers. CRSB explicitly forbids the use of these chemicals
for pharmaceutical use, illegal drug manufacture, explosives production, or
human consumption. They have a webpage at http://www.sb.net/wirehead.

Orders over US$100 must be made by company or personal check; orders under
US$100 may be made by money order (i.e., no paper trail). These aren’t
CRSB’s rules of choice, they’re California law, so don’t complain the CRSB
about it. I

A short disclaimer here: I have had no dealings with CRSB, and cannot
guarantee anything about them. Nobody on the Usenet drugs newsgroups who
has dealt with them has ever said anything negative about them (other than
their prices, which are understandable given the amount of legal paperwork
CRSB evidently goes through to be able to sell to researchers not
associated with established institutions). My personal belief is that they
are legit, and that in the case of ordering DXM it probably doesn’t matter
anyway, since DXM is neither scheduled nor prescription. Just remember that
this is for research purposes only.

Prices on DXM HBr have been decreasing, and CRSB runs specials, so check
their website for current pricing. The last time I checked, the prices were
10g for $92, 100g for $310. These prices are a bit stiff, but CRSB goes
through tremendous legal hassles to make numerous useful chemicals
available. All prices are US dollars.